In general, the energy level of emitted x-rays from an x-ray machine varies directly with the high voltage potential across the x-ray tube. The x-ray machine output is usually rated in terms of the maximum applied potential (kVp), which corresponds to the highest energy x-ray photons emitted from the source, since the highest energy photons determine the effective penetration of the x-ray beam.
The actual kVp applied to the x-ray tube may not be equal to the kVp setting indicated on the control panel of the x-ray machine. Thus it is useful to periodically check the accuracy of the kVp setting and recalibrate if necessary. It is difficult to directly measure the voltage applied across the x-ray tube, therefore it is preferable to take measurements of the x-ray beam, and from those measurements deduce the kVp.
One device, the kVp meter, utilizes a pair of x-ray detectors located behind filters of different filtration characteristics. The kVp may be determined from the ratio of the output signals of the two detectors. The detectors are typically semi-conductor photodiodes placed closely adjacent to one another to sample the same portion of the x-ray beam. One such digital kVp meter is disclosed in the specification of U.S. Pat. No. 4,697,280 to Zarnstorff and Ranallo.
It has been found useful to design detectors such that the center of the area of both detectors is the same, thereby eliminating cathode-anode dependency of the meter.
It has also been found necessary to amplify the output of the detectors, to allow easier comparison of the outputs. The outputs of the detectors may vary over a wide range, based on the kVp selected, as well as the mA selected. Due to this wide range of output levels, it is necessary to have an adjustable gain amplifier. This may be accomplished by having a circuit that adjusts the gain based on the output of the amplifier, wherein the gain is initially at the maximum and is adjusted downward until the output of the amplifier falls below a given level.
Prior art devices have either sampled and summed or integrated the amplified outputs of the detectors over time. The ratio of the first integrated or summed value to the second integrated or summed value is taken and the kVp is determined from this ratio. Corrections are made for different types of x-ray machines. Since the data was summed or integrated, the actual parameter measured was the average or effective kVp. In other words the meter smooths out the ripple present in any non-constant potential x-ray unit and does not measure actual peak kV.